The present invention relates to a process for the acoustic examination of monoliths for damage as well as a device for carrying out the process.
Monolithic carrier elements, as are used, for example, during the manufacture of waste gas catalysts and soot filters for motor vehicles, consist usually of relatively porous ceramic materials. The considerable porosity of the material used leads to possible damage of the monoliths during their manufacture or further processing; in particular, during handling of a monolith, fine cracks can form at the latter's surface or its interior. Such cracks render the carrier element practically unusable, since they constitute a starting point for a possible break of the monolith at a later time; a destruction of such cracked monoliths may take place already during relatively small loads, for example, due to jarring during their processing and/or during operation. For this reason, monolithic carrier elements must not be processed further, even if they have been damaged only slightly.
Therefore, it is required to examine carrier elements for possible damage prior to further processing. In addition to an optical examination, which naturally can address merely the condition of the surface, an acoustic examination takes place as well. For this purpose, a trained person lightly taps the monolith and evaluates the noise subsequently emitted by the monolith. For this purpose, the understanding is taken into consideration that cracks in the interior of the monolith influence the latter's vibration behavior and thus the radiated frequencies.
With such an acoustic examination, various disadvantages are connected: For one thing, the reproduceability is not satisfactory, because the subjective sound perception by the person conducting the test is critical to the result of the examination. Also, the striking location and strength, carried out manually in the known process for acoustic examination, influence the result of the examination inasmuch as the resulting variation in loudness of the radiated sound alters the sound perception of the examining person. Thus, all in all, a low degree of reliability results from the conventional acoustic examination, which in practice is expressed by a large portion of monoliths which can neither be clearly classified as flawless nor clearly as damaged. After, for safety reasons, only those monoliths are processed further, which have been classified as clearly flawless, the shown unreliability of the testing method used leads to a relatively high rejection rate during the production and thus to higher manufacturing costs.